1. Field of the Invention
The present invention relates to an image forming apparatus utilizing, for example, an electrophotographic or electrostatic recording process, and more particularly to an image forming apparatus such as a copier, a printer or a FAX machine.
2. Description of the Related Art
An electrophotographic process is one of the most known printing processes often utilized in copiers and printers. Recently, more attention upon POD (print on demand) has increased demands for a printing capability at higher speed, photographic image printing, etc. As a result, printers capable of producing finer images with higher quality have been demanded.
In general, developing devices equipped in image forming apparatuses utilizing the electrophotographic or electrostatic recording process employ a one-component developer containing a magnetic toner as a main component or a two-component developer containing a non-magnetic toner and a magnetic carrier as main components. In color image forming apparatuses utilizing the electrophotographic process to form full-color and multicolor images, especially, most of developing devices employ a two-component developer for the purpose of obtaining more satisfactory color tints of images.
As well known, a toner density (i.e., a ratio of toner weight to total weight of the carrier and the toner) of the two-component developer is a very important factor from a point of stabilizing image quality. The toner in the developer is consumed in a developing step, and the toner intensity of the developer reduces correspondingly. It is therefore necessary to detect the toner (developer) density or the image density at the appropriate times by using a developer density controller or an image density controller, and to replenish the toner depending on a detected density change so that the toner density or the image density is always controlled to be constant and image quality is held at a satisfactory level. FIG. 2 shows an example of overall construction of an image forming apparatus, e.g., an electrophotographic digital copier, equipped with a known density controller.
First, an image of a document 31 is read by a CCD 1. A resulting analog image signal is amplified to a predetermined level by an amplifier 2 and converted into a digital image signal of, e.g., 8 bits (0 to 255 levels of halftone) by an analog-digital converter (A/D converter) 3. Then, the digital image signal is supplied to a γ-converter (which is constructed as a 256-byte RAM and performs density conversion using a lookup table in this example). Further, the digital image signal is subjected to γ-compensation and inputted to a digital-analog converter (D/A converter) 9.
The digital image signal is converted again into an analog image signal by the D/A converter 9 and supplied to one input terminal of a comparator 11. A triangular wave signal generated from a triangular wave generator 10 and having a predetermined period is supplied to the other input terminal of the comparator 11. The analog image signal supplied to the one input terminal of the comparator 11 is compared with the triangular wave signal for pulse width modulation. A binary image signal having been subjected to the pulse width modulation is inputted, as it is, to a laser driver 12 and used as an on/off control signal for causing a laser diode 13 to emit a laser beam. The laser beam emitted from the laser diode 13 is scanned by a known polygonal mirror 14 in the direction of main scan and is illuminated through an f/θ-lens 15 and a reflecting mirror 16 onto a photoconductive drum 40. The photoconductive drum 40 serves as an image carrying member and is rotated in the direction denoted by an arrow (a in FIG. 3). An electrostatic latent image is thus formed.
On the other hand, after uniform charge cancellation by an exposure device 18, the photoconductive drum 40 is uniformly charged to be, e.g., negative by a primary charger 19. Then, the photoconductive drum 40 is exposed to the illumination of the laser beam; as described above, whereupon an electrostatic latent image is formed in accordance with the image signal.
The electrostatic latent image is developed into a visible image (toner image) by a developing device 20. A toner replenishing tank 8 containing a make-up toner 29 is provided above the developing device 20, and a toner feed screw 30 is mounted at the bottom of the toner replenishing tank 8. The toner feed screw 30 is rotated by a motor 28 to feed the toner 29 for supply into the developing device 20.
The toner image formed on the photoconductive drum 40 is transferred under an action of a transfer charger 22 onto a transfer material P, which has been transported to the photoconductive drum 40 by a transfer material carrying belt 17. The transfer material carrying belt 17 is stretched between two rollers 25a and 25b and is endlessly driven to move in the direction denoted by an arrow in FIG. 2, thus transporting the transfer material P held on it to the photoconductive drum 40. The toner remaining on the photoconductive drum 40 after the image transfer is scraped off by a cleaner 24.
For simplicity of the explanation, FIG. 2 shows only a single image forming station (including the photoconductive drum 40, the exposure device 18, the primary charger 19, the developing device 20, etc.). In the case of a color image forming apparatus, however, image forming stations corresponding to respective colors, e.g., cyan, magenta, yellow and black, are successively arranged along the transfer material carrying belt 17 in the direction of movement thereof.
FIG. 3 shows one example of the developing device 20.
In the example of FIG. 3, the developing device 20 comprises a development container 52 containing a two-component developer (21 in FIG. 2), and a development sleeve 58 serving as a developer carrying member and rotatably mounted in the development container 52 with a predetermined gap left relative to the photoconductive drum 40. The development sleeve 58 is constituted as a cylindrical member made of a non-magnetic material, and a magnet roller 59 serving as a magnetic field generating means is disposed inside the development sleeve 58 to be held stationary with respect to the rotation of the development sleeve 58 denoted by an arrow c. The magnet roller 59 has five magnetic poles N1, S1, N2, N3 and S2. A restriction blade 131 as a magnetic member is attached to a portion of the development container 52 positioned above the development sleeve 58. The restriction blade 131 is disposed in non-contact relation to the development sleeve 58 such that its lower end is extended to and located near the magnetic pole S2 which is positioned substantially at a top point of the magnet roller 59 in the vertical direction. A pair of developer feed screws 54, 56 are disposed in a lower portion of the development container 52.
The two-component developer contained in the development container 52 is supplied to the development sleeve 58 while circulating in the development container 52 with agitating and feeding actions of the developer feed screws 54, 56. The developer supplied to the development sleeve 58 is drawn up onto the development sleeve 58 by an action of the magnetic pole N3 of the magnet roller 59. With the rotation of the development sleeve 58, the developer is carried over the development sleeve 58 from the magnetic pole S2 to the magnetic pole N1 and then reaches a developing area where the development sleeve 58 and the photoconductive drum 40 are positioned to face each other. While being carried to the developing area, a layer thickness of the developer is magnetically restricted by the restriction blade 131 in cooperation with the magnetic pole S2 so that a thin layer of the developer is formed on the development sleeve 58.
The magnetic pole N1 of the magnet roller 59, positioned in the developing area, serves as a main pole for development. The developer carried to the developing area is heaped up by an action of the magnetic pole N1 and comes into contact with the surface of the photoconductive drum 40, whereby the electrostatic latent image formed on the surface of the photoconductive drum 40 is developed. After developing the latent image, the developer exits the developing area with the rotation of the development sleeve 58 and is returned to the development container 52 through the magnetic pole S1 serving as a carrying pole. The developer is then removed from the development sleeve 58 for recovery under repulsive magnetic fields produced by the magnetic poles N2, N3.
Such an image forming apparatus includes any type of density controller (ATR (Auto Toner Replenishment) unit) for the purposes of controlling replenishment of the toner to the developer 21 in the developing device 20 in which the toner density has reduced with repetition of the developing step described above, and controlling the toner density of the developer or the image density to be kept constant.
In practice, there are known a control method of detecting the toner density of the developer 21 in the developing device 20 based on the intensity of light reflecting from the developer by using a toner density sensor 23 mounted in the developing device 20 (called “developer reflection ATR”), a control method of forming a reference patch image 26 on the photoconductive drum 40 and detecting the density of the patch image 26 by a sensor 27, e.g., a potential sensor, disposed in opposite relation to the photoconductive drum 40 (called “patch check ATR”), and a control method of computing the amount of required toner from a level of a digital image signal for each pixel output from a video counter 4 (called “video counter ATR”).
In any of those control methods, based on the obtained information, a CPU 6 controls rotation of a motor 28 through a motor driver 7. Correspondingly, replenishment of the toner to the developer 21 in the developing device 20 is controlled so as to keep constant the toner density of the developer or the image density.
Additionally, it is also proposed, for example, to correct an initial value used in the patch check ATR or the developer reflection ATR based on a result of the patch check, and to correct the amount of replenished toner based on a result of the patch check at the appropriate times while mainly employing the video counter ATR.
In those control methods, generally, the amount of replenished toner or a correction amount thereof is decided depending on the difference between an actual patch density and an initial patch density as shown in, by way of example, in FIG. 4. As the difference from the initial patch density increases, the amount of replenished toner is increased.
More specifically, according to any of those control methods, when the patch density is determined to be high (dark), control is performed to provide a proper patch density with consumption of the toner through the image forming process. On the other hand, when the patch density is determined to be low (light), control is performed to provide a proper patch density by directly replenishing the toner or correcting an initial value in the developer reflection ATR, etc.
Prior to the start of such control, it is preferable to experimentally confirm the amount of replenished toner with respect to the amount of rotation of the toner feed screw 30.
In two-component developing devices practiced so far, developer density controllers utilizing methods of directly measuring the toner density (e.g., the so-called optical ATR and inductance control) are intended to primarily realize stability in the toner density. Therefore, the toner density is stabilized, but those controllers cannot follow a variation in the amount of toner charge (hereinafter also referred to as “tribo-charge”) caused by, e.g., changes in the charging capability of the carrier, non-operation for a long time, and abrupt changes in ambient environment of the image forming apparatus. As a result, fluctuations beyond an allowable range may occur in color tint and image density. There is hence still room for improvement in that type of controller.
Meanwhile, the so-called patch check ATR has also hitherto been practiced for the purpose of keeping constant the amount of toner developed on the photoconductive drum. The patch check ATR comprises the steps of forming a predetermined reference patch on the photoconductive drum at the appropriate times, comparing the detected density of the reference patch with the initial density, and executing toner replenishment control in accordance with a result of the comparison.
The patch check ATR has two functions of avoiding fluctuations in image density and color tint and performing toner density control by keeping constant the toner amount of the reference patch on the photoconductive drum. Also, keeping constant the toner amount of the reference patch on the photoconductive drum by the patch check ATR contributes to reducing fluctuations in the amount of toner charge and hence stabilizing the electrostatic transfer subsequent to the developing step. Consequently, the image density can be held in an allowable fluctuation range, and fluctuations in the color tint can also be held down small.
However, the patch check ATR control may cause extreme fluctuations in the toner density and bring about a trouble because the toner density is controlled to suppress a variation in the amount of toner charge caused by, e.g., changes in the charging capability of the carrier, non-operation for a long time, and abrupt changes in ambient environment of the image forming apparatus, aiming at control to keep constant the image density of the reference patch. For example, when the toner density extremely increases, toner scattering may occur, and when the toner density extremely decreases, a coarse or rough image and carrier attachment may occur.
Furthermore, in apparatuses disclosed in Japanese Patent Laid-Open Nos. 08-110700, 10-039608, and 2001-296732, it is proposed to perform toner replenishment control of deciding the amount of replenished toner from an output of a device (e.g., a photosensor or inductance sensor) for directly measuring the toner density as described above, to form a reference patch image at the appropriate times, and to correct the amount of replenished toner, which has been determined based on the output of the toner-density directly measuring device, in accordance with a detected density of the reference patch image.
However, such a method is intended for the toner replenishment control primarily aiming at stabilization of the toner density in the developing device, and does not take into account fluctuations in the image density and the color tint as main factors. For that reason, the disclosed apparatuses also still have room for improvement.